1. Field of the Invention
This invention relates to a hydraulic antivibration device, more particularly to such a hydraulic type of antivibration device that is capable of restraining the generation of strange or unusual sounds (noise) while ensuring a low dynamic spring characteristic upon inputting of vibrations of a relatively small amplitude and a high damping characteristic upon inputting of vibrations of a relatively large amplitude.
2. Description of Related Art
As an antivibration device supporting and securing an automotive engine so as not to transmit the engine vibration to the body frame, a hydraulic type (liquid-filled type) of one is known.
A typical hydraulic antivibration device is constructed so that a first attachment fitting to be mounted to the engine side and a second attachment fitting to be mounted to the body frame side are connected through a vibration-isolating base composed of rubber-like elastomer, and a liquid-filled chamber is formed between a diaphragm attached to the second attachment fitting and the vibration-isolating base. And the liquid-filled chamber is comparted by a partition member into a first and a second liquid chambers, which are in turn brought into communication with each other through an orifice.
According to this hydraulic antivibration device, it is possible to perform a vibration damping function and a vibration isolating function owing to a fluidization effect of fluid between the first and second liquid chambers through the orifice and a vibration-deadening effect of the vibration-isolating base.
For such hydraulic type antivibration devices, further known are a so-called elastic membrane structure, wherein an elastic partition membrane is disposed between the first and second liquid chambers so that a hydraulic pressure variation (hydraulic pressure difference) between both chambers may be absorbed by deformation of the elastic partition membrane due to reciprocating motion thereby to obtain a low dynamic spring characteristic upon inputting of a small amplitude vibration (JP Patent Application Publication 2001-27277 A, e.g., FIG. 12), a so-called movable membrane structure, wherein displacement-regulating members are provided at both sides of the elastic partition membrane so that the displacement magnitude of the elastic partition membrane may be regulated from the both sides to increase the stiffness of the membrane thereby enhancing a vibration-damping characteristic upon inputting of a large amplitude vibration (JP Patent No. 2875723, e.g., FIG. 4), and so forth.
With the elastic partition membrane structure mentioned above, however, the problem of strange sounds, which will be later described, does not arise, yet the stiffness of the elastic partition membrane is constant irrespective of amplitudes, so that when a low dynamic spring characteristic will be obtained upon inputting of a small amplitude vibration, the hydraulic pressure difference between both liquid chambers is easy to be alleviated by the elastic partition membrane, upon inputting of a large amplitude vibration. As a consequence, the problem here was that it was unable to exhibit sufficiently the fluidization effect of fluid, causing a significant reduction in damping characteristic.
On the other hand, in the movable membrane structure, the opening area for transferring the hydraulic variation of the first liquid chamber or the second liquid chamber to the elastic partition membrane is narrowed by the space amount in which the displacement-regulating member is disposed. Thus it was unable to transmit the hydraulic pressure variation between both liquid chambers efficiently to the elastic partition membrane, with the result that the hydraulic pressure difference was difficult to be alleviated by the elastic partition membrane and hence it was hard to obtain a low dynamic spring characteristic.
Further problem with the movable membrane structure was that because of the structure that the elastic partition membrane is made to abut against the displacement-regulating member, upon abutting thereof the displacement-regulating member is oscillated, which oscillation is transmitted to the body frame, thereby generating strange sounds.